1. Field of the Present Invention
The present invention generally relates to system converter devices for video signals, and more particularly, to a system converter device for converting video signals having a certain number of scanning lines to video signals having a less number of scanning lines. The present invention has particular application to a system converter device for converting video signals based on the MUSE system to video signals based on the NTSC system.
2. Description of the Background Art
In the high definition television system, the number of scanning lines is 1125 lines per frame, the interlace ratio is 2:1, and the aspect ratio is 16:9. On the other hand, the NTSC system uses 525 scanning lines per frame, an interlace ratio of 2:1, and an aspect ratio of 4:3. Since one channel of satellite broadcasting has a bandwidth of 27 MHz, video signals based on a high definition television system are bandwidth compressed to the bandwidth of 8.1 MHZ. The bandwidth compressed signals are transmitted through one channel of the satellite broadcasting. This transmission system is called "Multiple Sub-Nyquist Sampling Encoding" (hereinafter referred to as MUSE).
This MUSE transmission system is defined as a multiplexed subsample transmission system using offset subsampling between two fields and frames. Line sequential time axis integration (TCI) is employed in the MUSE transmission system, where the red color difference signal R-Y and the blue color difference signal B-Y have the time axis compressed to 1/4. The compressed signals are time axis multiplexed at the horizontal blanking period of the luminance signal Y. Furthermore, the red color difference signal R-Y and the blue color difference signal B-Y are multiplexed line sequentially on odd number lines and even number lines, respectively.
When video signals of the high definition television system transmitted according to the MUSE transmission system, for example, are converted to video signals of the NTCS system, a conversion system is proposed in which the image portion of 1050 scanning lines and with the aspect ratio of 4:3 is extracted from the 1125 lines of scanning line signals according to a high definition television system having the aspect ratio of 16:9, and further removing one half of the 1050 lines, i.e., 525 lines.
However, when video signals are converted in such a manner, there was a problem that the image information on both sides of the display screen of the high definition television, that is to say, the right and left sides of the image that should be displayed on the display screen, for example, is lost.
More specifically, the video signal according to the high definition television system comprises a video region A and video regions B adjacent to both sides of region A on a display screen 71, as shown in FIG. 10A. In the conventional converting system mentioned above, only video region A of FIG. 10A is displayed as video region A' on a display screen 72, as shown in FIG. 10B. In other words, video regions B on display screen 71 are lost in display screen 72.